The present invention relates to cutoff knives for use in cutting material (e.g., corrugated material). Corrugated material flows out of a corrugator as a continuous sheet (i.e., a web). The web is typically 8 to 9 feet wide and is moving at approximately 1000 feet per minute. The continuous web is cut into individual sheets by a machine that utilizes counter-rotating cutoff knives.
Early conventional knives had straight edges that contacted each other while cutting the web of material. The constant impacting of the knives with each other resulted in excessive knife and equipment wear. Past attempts at improving the life of the knives included lubricating the knives with oil. However, getting oil on the web of material presented a cleanliness issue, especially for food applications. As such, utilizing lubricant to reduce wear of the knives is not an attractive option for many applications. In an attempt to improve the cut quality, a serrated edge on one blade was used to cut against a standard straight edge. This reduced the creation of long thin strips of paper resulting from a double cut of the inner flute material (i.e., angel hair).
Later conventional cutoff knife designs utilized serrated knives that did not physically contact each other while cutting the web of corrugated material. In the cutoff knife industry, these knives have been referred to as “non-contact knives.” The knives were aligned in such a manner that the serration tooth of one knife passed through the valley in the serration of the other knife. Utilizing non-contact knives successfully cut the web of material without the use of lubrication, and since the knives were not contacting each other, it also significantly reduced the wear on the knives and the equipment. However, as explained in detail below, these conventional serrated non-contact knives were unable to achieve a clean cut in many cases.
With reference to FIG. 2, a pair of conventional serrated cutoff knives 10A, 10B is illustrated with the knives 10A, 10B intermeshed. Each of the knives 10A, 10B has a serration 14 that includes a plurality of teeth 18 defined between valleys 22. Due to the method used in grinding the serration 14 into the knives 10A, 10B, the serration valleys 22 have a large radius and the teeth 18 have a flat tip 20. The mating of the tooth 18 of one knife 10A into the radiused valley 22 of the opposing knife 10B results in a variable gap 26 formed between the two knives 10A, 10B. The gap 26 includes a first clearance 30 at the tooth 18 side that is larger than a second clearance 34 at the tooth tip 20. Since a varying clearance gap 26 exists between the cutting edges of the knives 10A, 10B, the cut of corrugated material is not clean and results in fibers being pulled instead of cut. This created a “fuzzy” edge on the cut corrugated material that is a common problem referred to as “fiber pull.” The cutting quality becomes even more of a problem when the cutoff knives in FIG. 2 are utilized in a corrugated material application that requires reinforcement tape. In these instances, the reinforcement tape, which is made up of fibers and adhesive, does not cut cleanly due to the variable clearance gap 26 formed by the intermeshed serrations 14.
With reference to FIG. 3, another pair of conventional serrated cutoff knives 50A, 50B is illustrated with the knives 50A, 50B intermeshed. Each of the knives 50A, 50B has a serration 54 that includes a plurality of teeth 58 defined between valleys 62. Each tooth 58 includes a flat apex 66 while the valleys 62 include a radius. The mating of the flat tooth 58 of one knife 50A into the radiused valley 62 of the opposing knife 50B results in a variable gap 70 formed between the two knives 50A, 50B. When the conventional serrated cutoff knives 50A, 50B are mated together, the large flat 66 defines a first clearance 74 that is larger than a second clearance 78 at the tooth 58 side. While the knives of FIG. 3 allow for a smaller clearance along the tooth 58 sides than the knives of FIG. 2 (i.e., clearance 78 of FIG. 3 is smaller than clearance 30 of FIG. 2), there is still a large clearance 74 between the tooth flat 66 and the radiused valley 62. As such, since a varying clearance gap 70 exists between the cutting edges of the knives 50A, 50B, the cut of corrugated material is not clean and results in fibers being pulled instead of cut.